The present invention is directed to wireless communication and, more particularly, to paging a mobile station.
To conserve battery power, a mobile station, such in a cellular or personal communications system (PCS) network that employs Code Division Multiple Access (CDMA), may operate in a slotted mode when not handling telephone calls and when not receiving information about the network. While operating in the slotted mode, the CDMA mobile station periodically monitors only those time slots of a paging channel (PCH) that are assigned to it to determine whether a base station is attempting to deliver an incoming call or message to it. At other times, the mobile station remains in an idle or standby state. However, the paging channel is shared by many mobile stations and carries both time slotted and non-time slotted messages. Therefore, the paging channel may carry information not intended for the mobile station in its time slot. The mobile station remains on even when the time slot does not include information directed to it because the mobile station must monitor every time slot assigned to it and may be required to monitor up two time slots in every cycle. Thus, the mobile station consumes battery power though there is no useful information in the time slot.
To further conserve power, a quick paging channel (QPCH) has been added as part of the IS-2000 wireless communication standard. The quick paging channel is used by the base station to alert an idle mobile station to activate and receive information carried by the paging channel starting from the next assigned paging channel time slot of the idle mobile station. The quick paging channel is divided into time slots which may include two indicator bits, known as paging indicator (PI) bits, that are assigned to a respective mobile station according to a hashing function used by the base station and the mobile station. The mobile station turns on at the time it is to receive its assigned PI bits. When both PI bits have been set to the value ON by the base station, the mobile station remains active to receive slotted information transmitted over the paging channel. Alternatively, when one or both of the PI bits have been set to the value OFF, the mobile station returns to its idle until the next time that it is to receive PI bits.
However, the quick paging channel PI bits assigned to a specific mobile station may precede the corresponding paging channel time slot by as much as 100 ms. This means it is possible for the base station to turn on the two PI bits assigned to a respective mobile station, to indicate the presence of a slotted message in the paging channel, and then be unable to send the message in the corresponding paging channel time slot. As an example, the base station may need to transmit a system overhead message, a general page message or other non-time slotted messages during this time slot. Alternatively, a slotted message intended for another mobile station and that began in an earlier paging channel time slot may run over into the time slot assigned to the current mobile station. As a further alternative, one or more slotted messages that were intended for a mobile station that does not support a quick paging channel may be awaiting delivery because paging channel time slots in earlier cycles that would ordinarily be assigned to the mobile station were instead used for mobile stations that were quick paged. Thus, a mobile station that detected two PI bits that were set to the value ON and activates to receive a paging channel message may fail to receive a message, thereby consuming battery power without receiving a message.
Though algorithms exist for allocating paging channel slots that may more reliably assure that a mobile station that detects two ON PI bits actually receives slotted messages on the corresponding paging channel, such algorithms are typically complex and difficult to implement in real time. It is therefore desirable that the two PI bits be assigned in a manner that more reliably indicates that a slotted message is present on the corresponding paging channel slot.